1. Field of the Invention
The present invention relates to a thermal displacement correction device in a working machine, and particularly, to a thermal displacement correction device that notifies degradation in correction precision of a thermal displacement correction.
2. Description of the Related Art
In a working machine, a structure provided therein is expanded or contracted during a processing operation of the working machine due to various factors such as heat of a motor, friction heat of a feeding shaft, cutting heat of a cutting operation, and heat transferred by a temperature of a cutting fluid or various ambient temperatures. Here, a displacement in the relative position between a tool and a workpiece affects the processing precision of the working machine. In order to prevent degradation in processing precision caused by the displacement of the relative position due to the heat, there is a need to correct the relative position between the tool and the workpiece in response to the displacement amount. For that reason, the working machine is equipped with a device that estimates a thermal displacement amount in response to an environment temperature or heat transferred from the above-described heat generation source and corrects the relative position.
As a method of estimating the displacement of the relative position between the tool and the workpiece due to the heat, JP 2013-146823 A discloses a method of calculating a thermal displacement amount of a structure from the amount of heat generated or radiated when the working machine is driven or stopped.
Further, JP 10-138091 A discloses a method of determining a correction amount by using a displacement or a change in temperature detected by a displacement sensor or a temperature sensor for the purpose of obtaining higher correction precision. In the technique disclosed in JP 10-138091 A, it is determined that the thermal displacement amount is not sufficiently estimated when a change in thermal displacement amount calculated by a normal thermal displacement estimation method is larger than a setting value. Then, the thermal displacement amount is measured by the displacement sensor and the correction amount thereof is determined.
Since the displacement caused by heat changes due to various factors such as heat of a motor, friction heat of a feeding shaft, cutting heat of a cutting operation, and an influence caused by a temperature of a cutting fluid or various ambient temperatures, it is difficult to completely estimate the displacement amount by checking the heat generation amount from all heat generation sources and the heat transfer amount to the structure without any sensor. For that reason, there is a difference with respect to the actual thermal displacement amount during a correction.
Since the thermal displacement becomes complex as the temperature distribution in the structure changes, a difference between the thermal displacement estimation amount and the actual thermal displacement amount increases. Accordingly, the correction precision is degraded even when the thermal displacement is corrected. For that reason, there is a case where the expected correction precision may not be obtained even after the correction of the thermal displacement depending on the thermal displacement amount. At that time, there is a need to adjust the offset value of the coordinate of the tool by using a unit other than a thermal displacement correction unit.
In such a case, an operator examines the correction precision by stopping the processing operation. However, when there is not any method of determining whether the sufficient correction precision is maintained by the correction of the thermal displacement, the examination is performed every processing operation, and hence the processing efficiency is degraded.
When the temperature sensor or the displacement sensor is used, the correction precision may be improved. However, the use cause the problem of increase of cost by the sensor use, and protection of the sensor from the influence of chips or coolant. Further, there is a need to consider a case where sufficient correction precision may not be obtained in accordance with the measurement precision of the sensor.
However, there is no working machine having a structure for determining whether the correction precision is sufficient in the correction of the thermal displacement.